1. Field of the Invention
This invention relates generally to a method for removing metallic and sulfur impurities and other impurities from titanium oxide hydrolysate at atmospheric pressure and at a temperature of from 60.degree. C. to 120.degree. C. so that it can be used in the preparation of various titanates and catalysts which require low levels of impurities in the formulations. The sulfur bound to the titanium oxide hydrolysate is removed and replaced by more volatile impurities which are easily removed in low temperature calcination.
The sulfate process for the production of titanium dioxide results in the production of a titanium iron sulfate liquor. A titanium oxide hydrolysate slurry can be precipitated from this liquor. This slurry will have various metallic impurities (e.g. iron) and free sulfuric acid. These can largely be removed with repeated washing with water.
However, the titanium oxide hydrolysate slurry may contain up to 2% sulfur (S) by weight in the form of sulfuric acid or sulfate ion which is either chemically bound to or adsorbed by the titanium oxide hydrolysate. Repeated washing of the titanium oxide hydrolysate slurry has very little or no effect on the sulfuric acid or sulfate ion which is either chemically bound to or adsorbed by the titania.
Sulfur present in the titanium oxide hydrolysate slurry will be driven off by heating at high temperature. However, using high temperatures presents the problem of emissions of sulfur and various sulfur containing compounds into the atmosphere. This requires the utilization of some type of device to prevent these sulfuric compounds from going into the atmosphere. The preferred method is to remove all of the impurities in the titanium oxide hydrolysate slurry at relatively low temperatures in order to save energy.
Ammonium compounds can be used to remove the sulfur after the washing of the titanium oxide hydrolysate slurry. The ammonia can be removed at low temperature because of the relatively high volatility of ammonia. However, introducing the ammonia into the atmosphere is also objectionable from an environmental standpoint.
The use of chlorine containing compounds and the use of compounds which produce metal cationic impurities are also objectionable, because they are not volatilized at relatively low temperatures and are pollutants and may enter into reaction with components which are combined with the titanium oxide hydrolysate.
2. Prior Art
U.S. Pat. No. 2,771,345, issued on Nov. 20, 1956 to Tanner discloses a process for the preparation of titanium dioxide pigments. It discloses a process for removing metallic and phosphatic impurities from titania hydrolysate prior to calcination into titanium dioxide. The titania hydrolysate is first digested in water with an alkali which converts the phosphatic impurities to a water-soluble condition. Water washing is used to remove the soluble phosphatic impurities. The titania hydrolysate is then treated with a strong mineral acid, such as sulfuric acid, to restore it to a flocculated state. The mineral acid removes the sodium impurities, but does introduce other impurities. For example, when sulfuric acid is used, sulfur impurities are added to the titania hydrolysate. It is then calcined at high temperatures to titanium dioxide.
U.S. Pat. No. 3,658,539 issued on Apr. 25, 1972 to Dantro discloses the use of ammoniacal compounds to prepare titanium dioxide powder. In this process, a titanium sulfate-iron sulfate solution is hydrolyzed to form hydrous titanium dioxide. It is then filtered, bleached, and washed to remove the soluble iron salt. An aqueous slurry is made of the hydrous titanium oxide and treated with a sufficient amount of an ammoniacal compound to raise the pH to at least 6.0. The neutralized hydrous oxide is then washed to remove the ammonium sulfate formed. The hydrous oxide is then dried to form soft aggregates of crystals of titanium dioxide. Ammoniacal compounds used are ammonium hydroxide, ammonium carbonates and ammonia gas. The Dantro process was designed to reduce the sulfur content through the reaction of the hydrous titanium oxide with the ammoniacal compound. The Dantro process does result in some ammonium salt remaining as an impurity in the titanium dioxide.
U.S. Pat. No. 5,011,666, issued on Apr. 30, 1991 to Chao et al. discloses a process for purifying TiO.sub.2 ore which can be used to make pigment or titanium metal. Chao et al. purifies the TiO.sub.2 ore by subjecting it to leaching treatments which involve alternating between the use of a mineral acid and an aqueous solution of an alkali metal compound. This treatment is designed to solubilize the iron, alkali metal, alkaline earth metal, rare earth metals, aluminum, phosphorous, thorium, uranium, chromium, manganese, silicon, vanadium, and yttrium impurities. These leaching processes have to be conducted at a temperature, pressure and time sufficient to solubilize the mineral impurities. Elevated temperatures and/or pressures are necessary. These impurities form a leachate which can be removed from the ore and washed with water. His process is not designed to drive off any sulfur that may be present, but rather the impurities named above.